What are the typical symptoms, diagnostic criteria, and first‑line treatment for small‑intestinal bacterial overgrowth (SIBO)?

Small Intestinal Bacterial Overgrowth (SIBO): Symptoms and Management

Clinical Presentation

Bloating and abdominal distention that worsen after meals are the hallmark symptoms of SIBO, typically accompanied by diarrhea, flatulence, and abdominal pain. 1

Core Symptoms

• Bloating and abdominal distention are the most characteristic features, with postprandial worsening 1
• Diarrhea occurs frequently due to bacterial fermentation and bile salt deconjugation 2, 3
• Flatulence and excessive gas production result from bacterial fermentation of carbohydrates 2
• Abdominal pain is common but nonspecific 3

Nutritional Consequences

• Fat-soluble vitamin deficiencies (A, D, E, K) develop from bile salt deconjugation and fat malabsorption 1
• Vitamin B12 deficiency occurs through bacterial consumption and impaired absorption 1
• Weight loss and frank malnutrition can occur in severe cases from macronutrient malabsorption 2, 3

Diagnostic Approach

The British Society of Gastroenterology recommends testing rather than empirical treatment to establish the diagnosis and support antibiotic stewardship. 4

First-Line Diagnostic Testing

• Combined hydrogen and methane breath testing (using glucose or lactulose) is more accurate than hydrogen-only testing and should be the initial diagnostic approach 4, 1
• Breath tests measure bacterial fermentation products and are noninvasive, accessible, and cost-effective 2

Alternative Diagnostic Methods

• Qualitative small bowel aspiration during upper endoscopy can be performed when breath testing is unavailable 4, 1:
  • Flush 100 mL sterile saline into the duodenum
  • Aspirate ≥10 mL into a sterile trap after a few seconds
  • Positive aspirates will grow colonic bacteria 4
• Traditional quantitative aspiration (>10⁵ CFU/mL) is time-consuming but remains the gold standard 2, 3

Common Pitfall

Avoid empirical antibiotic treatment without testing, as lack of response may indicate resistant organisms, absence of SIBO, or coexisting disorders causing similar symptoms 4

First-Line Treatment

Rifaximin 550 mg twice daily for 1-2 weeks is the most effective treatment for SIBO, achieving symptom resolution in 60-80% of patients with proven disease. 4, 1

Why Rifaximin is Preferred

• Non-absorbed from the GI tract, reducing systemic antibiotic resistance risk 4, 1
• Most extensively studied antibiotic for SIBO with consistent efficacy data 4

Alternative Antibiotic Options

When rifaximin is unavailable or ineffective, use these equally effective alternatives 4, 1:

• Doxycycline (standard dosing for 1-2 weeks)
• Ciprofloxacin (use lowest effective dose; monitor for tendonitis) 4, 1
• Amoxicillin-clavulanic acid 4
• Cefoxitin 4
• Metronidazole has lower documented efficacy and should be avoided as first-line 4, 1

Special Consideration: Hydrogen Sulfide-Producing SIBO

For hydrogen sulfide-producing SIBO, combine bismuth subcitrate 120-240 mg four times daily (30 minutes before meals) with rifaximin 550 mg twice daily for 14 days 1

• Avoid bismuth use beyond 6-8 weeks continuously to prevent neurotoxicity 1

Management of Recurrent SIBO

For patients with reversible causes (e.g., immunosuppression during chemotherapy), one antibiotic course is typically sufficient; for recurrent SIBO, use cyclical antibiotics, rotating 1-2 week treatment periods with antibiotic-free intervals. 4, 1

Strategies for Recurrence

• Cyclical antibiotics: Rotate antibiotics with 1-2 week periods without antibiotics before repeating 1
• Low-dose long-term antibiotics in select cases 4
• Recurrent short courses as needed 4

Address Underlying Causes

• Discontinue proton pump inhibitors immediately if they are contributing to SIBO; consider H2-blockers (famotidine) as safer alternatives if acid suppression is required 1
• Evaluate for impaired gut motility from diabetes with autonomic neuropathy, which disrupts the migrating motor complex 1
• Screen for anatomical abnormalities including incompetent ileocecal valve or prior abdominal surgery 1, 5
• Assess for pancreatic exocrine insufficiency, which occurs in up to 92% of chronic pancreatitis patients and predisposes to SIBO 1, 5

Nutritional Management

Screen for and replace fat-soluble vitamins (A, D, E, K) in all patients with steatorrhea or confirmed SIBO. 1

Key Nutritional Interventions

• Monitor vitamin B12 levels and replace as needed, as bacterial overgrowth causes consumption and malabsorption 1
• Vitamin D deficiency occurs in 20% of patients taking bile acid sequestrants 4
• Consider bile salt sequestrants (cholestyramine or colesevelam) if steatorrhea persists after antibiotic treatment 1

Treatment Monitoring

If symptoms persist after completing antibiotic treatment, repeat breath testing to confirm SIBO eradication before considering alternative diagnoses. 1

Alternative Diagnoses to Consider

• Bile acid diarrhea may coexist or emerge after SIBO treatment 1
• Pancreatic exocrine insufficiency (faecal elastase <500 μg/g) can mimic or coexist with SIBO 4, 1
• Inflammatory bowel disease coexists with SIBO in 30% of Crohn's disease patients 1

Important Clinical Note

SIBO is not contagious and cannot be transmitted person-to-person, as it develops from overgrowth of bacteria already present in the GI tract due to underlying predisposing factors 5